In the early days of the Post Office, most Postmasters were local shop owners; the Post Office was located in their shop. If another shop owner became the Postmaster, the Post Office would move to that person's shop.

Alvarado

The first Sanborn map of Alvarado is from 1887. Before this, locations of the Post Office can only be guessed based on who was postmaster and where their business was located. From 1853 to 1859, the Post Office was probably located in the same building as the original County Courthouse. This building was owned by Henry Smith and Augustus Church, who were the first and second postmasters, respectively.

The 1887 Sanborn map shows the Post Office in the building just to the left of the Odd Fellows Hall on Smith Street. At this time, Edwin R. Richmond was the postmaster. The Post Office is shown as only part of the building; the largest part is marked as a dwelling. It looks like Edwin probably ran the Post Office from part of his home.

The 1908 Sanborn map shows a Post Office as part of the John Ralph store on Levee Street (now Union City Blvd), but Edwin Richmond was serving his second term as Postmaster. From 1899 to 1903, I. V. Ralph was postmaster and probably moved the Post Office to his store which was later sold to his nephew, John Ralph. In 1911, John Ralph became the Postmaster and the Post Office stayed in his building.

The 1925 Sanborn map shows that the Post Office had moved to a building across from the Odd Fellows Hall on Smith Street, in the same building as the library. Once a library was started, it was common to have the library and post office in the same building, the Postmaster also being the librarian. This meant that the post office had a permanent home. The post office probably moved to this location when Albert Norris became Postmaster in 1915 and remained there for the tenure of the next three postmasters.

In 1952, a new building was built for the post office and library at 3861/3851 Smith Street. The Alvarado Post Office Station still resides in the building. When the Union City Library was created in 1965, the Alvarado Library closed.

Decoto

The 1908 Sanborn map shows the Post Office in the John L. Olsen's Market at the corner of 10th and I Streets. In 1903, John Olsen was appointed Postmaster and held that position until 1936. There were seven earlier postmasters for Decoto. The second postmaster, Edward Salz, owned a number of buildings and a hay and feed business in Decoto. It can be surmised that one of them housed the post office. There is no documentation of where earlier post offices were located.

In 1936, Mary Janiero (married name: Mary George) became postmaster. She kept the post office in one half of the market until the 1950s when it was moved to a building at the corner of Second and E streets. Following the incorporation of Union City, Mary became the first Union City Postmaster and in 1965, a new post office was built at 33641 Mission Blvd. In 1983, the post office moved to its current location on Alvarado-Niles Road.